In certain wired and wireless communication systems or networks, devices may transmit data over a communications channel (e.g. a band or channel of spectrum, also referred to as a “medium”) only after sensing that the channel is not in use (“clear” or “idle”). However, if a plurality of devices that share a channel (for example, devices in a network) each try to transmit immediately after sensing that the channel is not currently in use, all the devices that were waiting for a clear channel may try to transmit at the same time immediately after the channel ceases to be busy. The resulting “collision” between the signals can prevent one or more of the devices from making a successful transmission.
To reduce the chance of such collisions, some wireless communication standards define a “Contention Window” (CW) scheme including a contention period, during which devices that want to transmit will wait, after sensing an open communications channel, before actually performing a transmission. The devices may wait for one or more additional periods along with the contention period. The total amount of time that the devices have to wait (i.e. delay their transmission) can be known as a “back-off period”. According to one contention-based scheme, each device may choose a time period randomly, and wait until the channel has been idle for this time period before trying to transmit. The CW helps define the maximum period that the device should wait, e.g., the random time periods are chosen to be within the CW (e.g. between 0 and CW). If the resulting first transmission attempt is unsuccessful, the length of the contention, window can be repeatedly increased for subsequent retries, up to some maximum value, until a retry is successful, or until a maximal number of retransmissions is reached. The length of the contention window may be defined by a first value, denoted CWmin, which defines a maximum initial starting size of the CW (the initial length of the CW being randomly chosen between 0 and CWmin), and a second value, denoted CWmax, which defines the maximal size of the CW.
In some wireless networks, all devices including an access point (AP) contend for the medium. In some wireless communications standards, the AP has more favourable medium access parameters than other devices, e.g. non-AP stations (STAs). However, even the more favourable medium access parameters can be insufficient to mitigate the statistical probability of a STA gaining access to the medium before the AP in the case where there are many STAs contending for the medium. This can result in lowered throughput from the AP to STAs.
Thus there is a need for a mechanism that can help improve the throughput from the AP to STAs when there are many STAs contending for the medium. Additionally, there is need to improve the throughput from the STAs to the AP when there is a low number of STAs contending for the medium. There may also be a need to improve or control the throughput (in either direction) when factors other than the number of STAs in the network affect the throughput. Embodiments of the present invention can help to provide devices, methods and computer programs for transmitting data on a communications channel which, at least partially, can help overcome the abovementioned problems.